Beam-forming is a spatial filtering technique for transmitting signals in a desired direction or for receiving only signals transmitted in a desired direction, using a plurality of transmission/reception antennas. A method of forming beams in a desired direction includes a switched beam-forming method and an adaptive beam-forming method. The switched beam-forming method forms beams by setting weight vectors for several directions, and the adaptive beam-forming method updates weight vectors according to a user's positions.
By using such a beam-forming technique, Spatial Division Multiple Access (SDMA) can be implemented in such a manner that the range of a cell is increased or the same frequency is allocated to different users in different directions, in a wireless communication system.
Meanwhile, since a wireless communication system requires a very high channel capacity in order to transmit data at a high speed, a Multiple Input Multiple Output (MIMO) system has been developed to satisfy such a high channel capacity. In the MIMO system, generally, different transmission antennas transmit different types of information in order to increase the amount of information that is to be transmitted, or diversity is applied to transmission information in order to enhance reliability of information. Signals which are transmitted through a MIMO system undergo fading that varies depending on the spatially different paths of the signals, according to scatterers on a wireless channel, so that the signals have different spatial characteristics.
In the beam-forming system, a plurality of antennas receive signals with time differences according to the antennas' locations. The time differences are expressed by so-called steering vectors that represent the characteristics of the antennas in specific directions.
In order to form a beam in the beam-forming system, the antennas must have correlation and a distance (called “Nyquist space”) between two antennas, and the distance has to be smaller than λ/2. However, if the antennas are installed at intervals which are narrower than λ/2, spatial diversity may not be utilized in the MIMO system. This is because antennas have to be spaced by about 10 through 20λ in order to utilize spatial diversity.